Biogeosciences, 15, 5715–5731, 2018https://doi.org/10.5194/bg-15-5715-2018© Author(s) 2018. This work is distributed underthe Creative Commons Attribution 4.0 License.

On the formation of hydrothermal vents and cold seeps in theGuaymas Basin, Gulf of CaliforniaSonja Geilert1, Christian Hensen1, Mark Schmidt1, Volker Liebetrau1, Florian Scholz1, Mechthild Doll2,Longhui Deng3, Annika Fiskal3, Mark A. Lever3, Chih-Chieh Su4, Stefan Schloemer5, Sudipta Sarkar6, Volker Thiel7,and Christian Berndt1

1GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstraße 1–3, 24148 Kiel, Germany2Faculty of Geosciences, University of Bremen, Klagenfurter-Straße 4, 28359 Bremen, Germany3Department of Environmental Systems Science, ETH Zurich, Universitätstrasse 16, 8092 Zurich, Switzerland4Institute of Oceanography, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan5Federal Institute for Geosciences and Natural Resources, Stilleweg 2, 30655 Hannover, Germany6Department of Earth and Climate Science, Indian Institute of Science Education and Research Pune,Dr. Homi Bhabha Road, Maharashtra-411008, India7Geobiology, Geoscience Centre, University of Göttingen, Goldschmidtstr. 3, 37077 Göttingen, Germany

Correspondence: Sonja Geilert (sgeilert@geomar.de)

Received: 8 January 2018 – Discussion started: 6 February 2018Revised: 4 September 2018 – Accepted: 5 September 2018 – Published: 27 September 2018

Abstract. Magmatic sill intrusions into organic-rich sedi-ments cause the release of thermogenic CH4 and CO2. Porefluids from the Guaymas Basin (Gulf of California), a sed-imentary basin with recent magmatic activity, were investi-gated to constrain the link between sill intrusions and fluidseepage as well as the timing of sill-induced hydrothermalactivity. Sampling sites were close to a hydrothermal ventfield at the northern rift axis and at cold seeps located up to30 km away from the rift. Pore fluids close to the active hy-drothermal vent field showed a slight imprint by hydrother-mal fluids and indicated a shallow circulation system trans-porting seawater to the hydrothermal catchment area. Geo-chemical data of pore fluids at cold seeps showed a mainlyambient diagenetic fluid composition without any imprint re-lated to high temperature processes at greater depth. Seepcommunities at the seafloor were mainly sustained by mi-crobial methane, which rose along pathways formed earlierby hydrothermal activity, driving the anaerobic oxidation ofmethane (AOM) and the formation of authigenic carbonates.

Overall, our data from the cold seep sites suggest thatat present, sill-induced hydrothermalism is not active awayfrom the ridge axis, and the vigorous venting of hydrothermalfluids is restricted to the ridge axis. Using the sediment thick-ness above extinct conduits and carbonate dating, we calcu-

lated that deep fluid and thermogenic gas flow ceased 28 to7 kyr ago. These findings imply a short lifetime of hydrother-mal systems, limiting the time of unhindered carbon releaseas suggested in previous modeling studies. Consequently, ac-tivation and deactivation mechanisms of these systems needto be better constrained for the use in climate modeling ap-proaches.

1 Introduction

Abrupt climate change events in Earth’s history have beenpartly related to the injection of large amounts of greenhousegases into the atmosphere (e.g., Svensen et al., 2004; Gut-jahr et al., 2017). Among the most prominent of these eventswas the Paleocene–Eocene Thermal Maximum (PETM) dur-ing which the Earth’s atmosphere warmed by about 8 ◦C inless than 10 000 years (Zachos et al., 2003). The PETM waspossibly triggered by the emission of about 2000 Gt of car-bon (Dickens, 2003; Zachos et al., 2003). The processes dis-cussed regarding the release of these large amounts of car-bon in a relatively short time are gas hydrate dissociation,volcanic eruptions as well as igneous intrusions into organic-rich sediments, triggering the release of carbon during con-

Published by Copernicus Publications on behalf of the European Geosciences Union.

5716 S. Geilert et al.: On the formation of hydrothermal vents and cold seeps

tact metamorphism (Svensen et al., 2004; Aarnes et al., 2010;Gutjahr et al., 2017).

The Guaymas Basin in the Gulf of California is consid-ered one of the few key sites to study carbon release in a riftbasin exposed to high sedimentation rates. A newly discov-ered vent field in the Guaymas Basin, which releases largeamounts of CH4 and CO2 up to several hundreds of metersinto the water column (Berndt et al., 2016), stimulated thediscussion on the climate potential of magmatic intrusionsinto organic-rich sediments (e.g., Svensen et al., 2004).

The Gulf of California is located between the Mexicanmainland and the Baja California peninsula, north of the EastPacific Rise (EPR; Fig. 1). The spreading regime at EPR con-tinues into the Gulf of California and changes from a ma-ture, open-ocean type to an early opening continental riftingenvironment with spreading rates of about 6 cm yr−1 (Cur-ray and Moore, 1982). Its spreading axis consists of twograben systems (northern and southern troughs) offset by atransform fault (Fig. 1). The Guaymas Basin, which is about240 km long, is around 60 km wide, reaches water depthsof up to 2000 m, and is known as a region of vigorous hy-drothermal activity (e.g., Curray and Moore, 1982; Gieskeset al., 1982; Von Damm et al., 1985). Hydrothermal activ-ity in the Guaymas Basin was first reported in the southerntrough (e.g., Lupton, 1979; Gieskes et al., 1982; Campbelland Gieskes, 1984; Von Damm et al., 1985). Here, fluids em-anate partly from black-smoker-type vents at temperaturesof up to 315 ◦C (Von Damm et al., 1985). The rifting envi-ronment in the Guaymas Basin shows a high sediment accu-mulation rate of up to 0.8–2.5 m kyr−1, resulting in organic-rich sedimentary deposits of several hundreds of meters inthickness (e.g., Calvert, 1966; DeMaster, 1981; Berndt et al.,2016). The high sedimentation rate is caused by high biolog-ical productivity in the water column and an influx of ter-rigenous matter from the Mexican mainland (Calvert, 1966).Sills and dikes intruding into the sediment cover have a sub-stantial impact on the distribution of heat flow, other environ-mental conditions, and thus early diagenetic processes withinthe basin (Biddle et al., 2012; Einsele et al., 1980; Kast-ner, 1982; Kastner and Siever, 1983; Simoneit et al., 1992;Lizarralde et al., 2010; Teske et al., 2014).

Magmatic intrusions and cold seeps at the seafloor wereobserved up to 50 km away from the rift axis, and a re-cently active magmatic process triggering the alteration oforganic-rich sediments and releasing thermogenic CH4 andCO2 was proposed by Lizarralde et al. (2010). These au-thors attributed elevated CH4 concentrations and tempera-ture anomalies in the water column to active thermogenicCH4 production driven by contact metamorphism. Accord-ing to Lizarralde et al. (2010) ongoing off-axis hydrothermalactivity may cause a maximum carbon flux of 240 kt C yr−1

through the seafloor into the ocean and potentially into the at-mosphere. However, modeling studies investigating the life-time of such sill-induced hydrothermalism show that initialCH4 and CO2 release is intense and vigorous but can de-

cline just as quickly (< 10 kyr) (Bani-Hassan, 2012; Iyer etal., 2017).

During the expedition SO241 by RV SONNE in June/ July2015 a new hydrothermal vent field was discovered at theflank of the northern trough (Fig. 1; Berndt et al., 2016). Thediscovered mound rises up to 100 m above the seafloor andpredominant black-smoker-type vents suggest similar end-member temperatures and geochemical composition foundat the southern trough (Von Damm et al., 1985; Von Damm,1990; Berndt et al., 2016). The hydrothermal vent systememits methane-rich fluids with a helium isotope signatureindicative of fluids in contact with mid-ocean ridge basalt(Berndt et al., 2016). On this cruise, we sampled this re-cently discovered hydrothermal vent field and some of theoff-axis seeps above sill intrusions described by Lizarralde etal. (2010). The aim of this study was to investigate the fluidand gas compositions of the off-axis seeps in order to identifythe influence of sill intrusions on fluid circulation, gas com-position, and the timing of hydrothermal activity. The overallmotivation was thus to explore the regional and temporal ex-tent of hydrothermal activity in the area and to provide betterconstraints on carbon release from sedimented ridge systems.

2 Materials and methods

2.1 Sampling devices and strategy

During the RV SONNE expedition SO241, seven sites acrossthe central graben of the Guaymas basin were investigated(Fig. 1). Site-specific sampling and data recording were per-formed using (1) a video-guided multi-corer (MUC), (2) agravity corer (GC), (3) temperature loggers attached to a GCor sediment probe, (4) a video-guided VCTD/Rosette wa-ter sampler, and (5) a video-guided hydraulic grab (VgHG).Sites were selected according to published data on the lo-cations of seeps (Lizarralde et al., 2010) and seismic dataacquired during the cruise (see below).

2.1.1 Seismic data recording

Seismic data were collected using a Geometrics GeoEelStreamer of 150 and 183.5 m length and 96 and 112 channels,respectively. Two generator-injector guns in harmonic mode(105/105 cubic inch) served as the seismic source. Process-ing included navigation processing (1.5625 m crooked linebinning), 20, 45, 250, and 400 Hz frequency filtering, andpost-stack Stolt migration with water velocity yielding an ap-proximately 2 m horizontal and 5 m vertical resolution closeto the seafloor.

2.1.2 Sediment and pore fluid sampling

At seepage and vent sites, the video-guided MUC was usedto discover recent fluid release, which was indicated by typ-ical chemosynthetic biological communities at the seafloor

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Figure 1. Sample locations in the Guaymas Basin, Gulf of California studied during RV SONNE expedition SO241. (a) Overview of stations(Seep sites, Smoker Site, and Slope Site). Black square indicates enlarged area in (b). Site DSDP 477 in the southern trough is shown forcomparison. (b) Enlargement of the sampling locations. Red circles refer to GC employments and yellow triangles to MUCs. Brown squareat Graben Site refers to water-column sampling and temperature measurements. Black lines refer to seismic profiles displayed in Fig. 2.(c) Enlargement of the Smoker Site sampling locations. Note the different scale compared to (a) and (b). Black arrow refers to the locationof the hydrothermal mound described in Berndt et al. (2016).

(microbial mats, bivalves, etc.; Sahling et al., 2002). How-ever, small-scale, patchy distributions of active seepage spotsand the visibility of authigenic carbonate concretions madeit difficult to select the best locations for coring. Hence, thecomparison of results from different seeps might be biasedin this regard, as not all seepage areas could be sampled attheir most active places. GC deployments were typically per-formed at sites initially investigated with the MUC video sys-tem or at the center of suspected seeps (based on bathymetryand seismic data).

In total, we present pore fluid and gas data collected at theseepage sites North (GC01, MUC11), Central (GC03, GC13,GC15, MUC04), and Ring (MUC05), one reference site (seeReference Site below; GC04, MUC02), and the hydrothermalvent field (Smoker Site; GC09, GC10, MUC15, MUC16).The Reference Site that did not show active seepage or faultsindicated by seismic data was chosen to obtain geochem-ical background values. In addition, the slope towards theMexican mainland was sampled as well (Slope Site; GC07)(Fig. 1, Table 1). Immediately after core retrieval, GCs werecut, split, and subsampled. Samples were transferred into acold lab at 4 ◦C and processed within 1 or 2 h. Pore flu-ids were obtained by pressure filtration (e.g., Jahnke et al.,1982). After MUC retrieval, bottom water was sampled andimmediately filtered for further analyses. The sediment wastransferred into a cold lab and sampling was executed in an

argon-flushed glove bag. Pore fluids were retrieved by cen-trifugation and subsequent filtration using 0.2 µm celluloseacetate membrane filters (e.g., Jahnke et al., 1982). Sedimentsamples (2 cm3) for hydrocarbon analyses were taken usingcut-off 3 mL syringes. All hydrocarbon samples were takenimmediately after sediment surfaces were exposed after corecutting or sectioning, ensuring minimal disturbance to sedi-ment surfaces prior to sampling and transferred to vials con-taining concentrated NaCl solution (as seen in Sommer etal., 2009). MUCs were extruded and sampled from the top.GCs were sampled at the bottom ends of 1-m core sections,either at the core catcher or at freshly cut section ends. Insome cases additional samples were taken from within GCcore sections by cutting the core liner with an oscillating sawand inserting cut-off syringes into the sides of core sections.

2.1.3 Sub-seafloor temperature measurements

Temperature gradients and thermal conductivity were mea-sured at the North Seep, Central Seep, Reference Site, andSmoker Site as well as along a transect across the newly dis-covered hydrothermal vent field and the rift valley (GrabenSite). Miniaturized temperature loggers (MTL) were at-tached to GCs or to a 5 m long sediment lance at a samplingrate of 1 measurement per second. The absolute accuracyof these temperature measurements was about 0.1 K and the

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5718 S. Geilert et al.: On the formation of hydrothermal vents and cold seeps

Table 1. Station list and site names of GCs and MUCs taken in the Guaymas Basin with corresponding water depths. Heat flow and temper-ature gradient data measured either attached to GCs or to a sediment probe.

Site Site name Latitude Longitude Water depth Temp. gradient Heat flow SR MAR(N) (W) (m) (K m−1) (mW m−2) (m kyr−1) (g cm−2 yr−1)

GCs

St.07-GC01 North Seep 27◦33.301′ 111◦32.882′ 1845 0.14 28 n.d. n.d.St.10-GC04 Reference Site 27◦26.531′ 111◦29.928′ 1846 0.22 140 n.d. n.d.St.09-GC03 Central Seep 27◦28.138′ 111◦28.420′ 1837 n.d. n.d. n.d. n.d.St.09-GC13 Central Seep 27◦28.193′ 111◦28.365′ 1838 0.16 113 n.d. n.d.St.72-GC15 Central Seep 27◦28.178′ 111◦28.396′ 1837 n.d. n.d. n.d. n.d.St.51-GC09 Smoker Site 27◦24.472′ 111◦23.377′ 1840 11 8069 n.d. n.d.St.58-GC10 Smoker Site 27◦24.478′ 111◦23.377′ 1845 10 6509 n.d. n.d.St.47-GC07 Slope Site 27◦24.412′ 111◦13.649′ 671 n.d. n.d. n.d. n.d.

MUCs

St.33-MUC11 North Seep 27◦33.301′ 111◦32.883′ 1855 n.d. n.d. 1.7a 0.05a

3.5a 0.15a

St.23-MUC05 Ring Seep 27◦30.282′ 111◦40.770′ 1726 n.d. n.d. 0.5 0.01St.15-MUC02 Reference Site 27◦26.925′ 111◦29.926′ 1845 n.d. n.d. 2.3 0.04St.22-MUC04 Central Seep 27◦28.165′ 111◦28.347′ 1839 n.d. n.d. 1.7 0.04St.65-MUC15 Smoker Site 27◦24.342′ 111◦22.970′ 1846 n.d. n.d. 1.8 0.05St.66-MUC16 Smoker Site 27◦24.577′ 111◦23.265′ 1842 n.d. n.d. 2.1′b 0.08′b

0.4+b 0.02+b

HF lance

St.60a – HF008_P03 Smoker Site 27◦24.273′ 111◦23.396′ 1840 4.6 3206 n/a n/aSt.60a – HF008_P01 27◦24.623′ 111◦23.626′ 1834 0.86 599 n/a n/aSt.60a – HF008_P02 27◦24.554′ 111◦23.512′ 1840 2.8 1953 n/a n/aSt.60a – HF008_P04 27◦24.408′ 111◦23.288′ 1849 2039 1427 n/a n/aSt.60a – HF008_P05 27◦24.341′ 111◦23.177′ 1852 1014 710 n/a n/aSt.60a – HF008_P06 27◦24.265′ 111◦23.082′ 1844 0.74 516 n/a n/aSt.60b – HF008_P07 27◦24.193′ 111◦23.956′ 1834 0.8 579 n/a n/aSt.60b – HF009_P04 27◦24.543′ 111◦23.351′ 1837 15 10 835 n/a n/aSt.60b – HF009_P01 27◦24.605′ 111◦23.317′ 1837 0.39 274 n/a n/aSt.60b – HF009_P02 27◦24.552′ 111◦23.347′ 1834 3451 2415 n/a n/aSt.70 – HF011_P01 Graben Site 27◦25.802′ 111◦25.486′ 1870 0.38 262 n/a n/aSt.70 – HF011_P02 27◦25.460′ 111◦24.946′ 2019 0.48 338 n/a n/aSt.70 – HF011_P03 27◦25.955′ 111◦24.493′ 2046 0.43 302 n/a n/aSt.70 – HF011_P04 27◦25.837′ 111◦24.951′ 2025 0.46 320 n/a n/a

Authigenic carbonate

St.56-VgHG-4 Central Seep 27◦28.181′ 111◦28.379′ 1843 n/a n/a n/a n/aAbbreviations: SR, Sedimentation Rate; MAR, Mass Accumulation Rate; n.d. not determined; n/a not applicable. a Sedimentation and mass accumulation rates at Station 33 of the 0–13 and13–18 cm layers, respectively. b Sedimentation and mass accumulation rates at Station 66 of the 0–7 and 7–17 cm layers, respectively.

temperature resolution was 0.001 K (Pfender and Villinger,2002).

Thermal conductivity was measured on recovered corematerial in close vicinity to the MTLs using the KD2 ProNeedle Probe instrument. For temperature measurementsobtained by a lance, a constant thermal conductivity of0.7 W m−1 K was assumed. Data processing was done ac-cording to Hartmann and Villinger (2002).

2.1.4 Water column sampling

Water samples were taken by using a Niskin water sampler(Rosette System), equipped with a video camera designedfor near-seafloor sampling (Schmidt et al., 2015) in order

to study water column chemistry (i.e., dissolved CH4) andoceanographic parameters (i.e., temperature, salinity, turbid-ity). Eight water sampling locations were chosen in the vicin-ity of MUC and GC stations and were termed the North Seep(VCTD03), Central Seep (VCTD02), Ring Seep (VCTD01),Graben Site (CTD01; no video-guided sampling), SmokerSite (VCTD06 and 10), and Slope Site (VCTD07). The(V)CTDs were either used in a towed mode (VCTD03, 06,09, 10) or in station (CTD01; VCTD01, 02, 07) keeping hy-drocast mode. The water depth was controlled based on pres-sure readings, altitude sensors (< 50 m distance to bottom),and online video observation (1–2 m above the seafloor).

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S. Geilert et al.: On the formation of hydrothermal vents and cold seeps 5719

2.1.5 Authigenic carbonate sampling

At the Central Seep a block (approx. 1× 0.5× 0.3 m) wasrecovered using a video-guided hydraulic grab (VgHG, GE-OMAR) in 1843 m water depth from the surface of a typi-cal cold seep environment (close to high abundance of tubeworms) . The block consisted mainly of a solidified carbonatematrix covered by a whitish carbonate rim and was charac-terized by coarse open pore space in the mm to cm scale (seeSupplement Fig. S1).

2.2 Sample treatment and analytical procedures

Pore fluids were analyzed onboard for total dissolved sulfide(TH2S) and NH4 directly after recovery by photometry usingstandard methods described in Grasshoff et al. (2002). Priorto NH4 measurements, pore fluids containing dissolved sul-fide were treated with argon to prevent biased NH4 measure-ments. Total alkalinity (TA) was determined by titration im-mediately after pore-water separation using 0.02 M HCl (Iva-nenkov and Lyakhin, 1978). Shore-based analyses of the re-maining acidified pore water included dissolved anions (SO4,Cl) and cations (Li, Mg) using ion chromatography (IC,METROHM 761 Compact, conductivity mode) and induc-tively coupled plasma optical emission spectrometry (ICP-OES, VARIAN 720-ES), respectively. All chemical analyseswere tested for accuracy and reproducibility using the IAPSOsalinity standard (Gieskes et al., 1991).

Strontium isotope ratios were analyzed by thermal ioniza-tion mass spectrometry (TIMS, Triton, ThermoFisher Scien-tific). The samples were chemically separated via cation ex-change chromatography using the SrSpec resin (Eichrom).The isotope ratios were normalized to the NIST SRM 987value of 0.710248 (Howarth and McArthur, 2004), whichreached a precision of ±0.000015 (2 sd, n= 12). Potentialinfluences of 87Rb interferences on 87Sr / 86Sr isotope ratioswere eliminated by combining the highly selective Sr-Specresin and Rb/Sr-discriminating TIMS preheating procedureswith the static mode measurement of 85Rb simultaneouslywith the Sr masses 84, 86, 87, and 88 for optional Rb/Sr cor-rections (not required in this study).

Water samples taken from Niskin bottles were transferredinto 100 mL glass vials with a helium headspace of 5 mL andpoisoned with 50 µL of saturated mercury chloride solution.

The hydrocarbon composition of headspace gases was de-termined using a CE 8000 TOP gas chromatograph equippedwith a 30 m capillary column (Restek Q-PLOT, 0.32 mm) anda flame ionization detector (FID). Replicate measurementsyielded a precision of < 3 % (2 sd).

Stable carbon isotopes of methane were measured us-ing a continuous-flow isotope-ratio mass spectrometer (cf-IRMS). A Thermo TRACE gas chromatograph was usedto separate the light hydrocarbon gases by injecting up to1 mL headspace gas on a ShinCarbon ST100/120 packedgas chromatography column. The separated gases were com-

busted and corresponding δ13C values were determined usinga Thermo MAT 253 mass spectrometer. The reproducibil-ity of δ13C measurements was ±0.3 ‰ (2 sd), based on re-peated measurements of the reference standard Vienna PeeDee Belemnite (VPDB).

Stable hydrogen isotope compositions of methane wereanalyzed by separating methane from other gases by onlinegas chromatography (Thermo Trace GC, isotherm at 30 ◦C;30 m RT-Q-Bond column, 0.25 mm ID, film thickness 8 µm).Methane-H was reduced to dihydrogen at 1420 ◦C prior tostable isotope analysis using a coupled MAT 253 mass spec-trometer (Thermo). Data are reported in per mil relative toStandard Mean Ocean Water (SMOW). The precision of δD-CH4 measurements was ±3 ‰ (2 sd).

210Pb (46.52 keV) and 214Pb (351.99 keV) were simulta-neously measured on freeze dried sediments by two HPGegamma spectrometry systems (ORTEC GMX-120265 andGWL-100230), each interfaced to a digital gamma-ray spec-trometer (DSPecPlus™). The efficiency calibration of thegamma detectors were calibrated using IAEA reference ma-terials (for GMX-type detector – 327A, 444 spiked soil, CU-2006-03 spiked soil, RGTh and RGU for sample weightat 100 g; for well-type detector – IAEA-RGTh and RGUfrom 0.5 to 3.5 g), coupled with an in-house secondary stan-dard (“Rock-falling Mountain soils”, Radiation MonitoringCenter of the Atomic Energy Council, Taiwan) for variousmasses (Lee et al., 2004; Huh et al., 2006). 214Pb was usedas an index of 226Ra (supported 210Pb) whose activity con-centration was subtracted from the total 210Pb to obtain theexcess 210Pb (210Pbex). The activities of radionuclides weredecay-corrected to the date of sample collection. All radionu-clide data were calculated on salt-free dry weight basis.

A representative sample of the authigenic carbonate (cm-scale) was broken from the upper surface of the block, gen-tly cleaned from loosely bound sediment and organic re-mains, and dried at 20 ◦C for 12 h. Two different subsampleswere prepared by drilling material with a handheld mm-sizedmini-drill from the outer rim (whitish coating, lab code: 470-15) and the related inner core (dark matrix, lab code: 472-15).

Prior to aliquot procedures both subsamples were finelyground in an agate mortar providing homogeneous aliquotsof suitable grain size for the mineral identification by X-ray diffractometry (XRD) (Philips X-ray diffractometer PW1710 in monochromatic CuKα mode between 2 and 702θ , incident angle, for details see Supplement). Subsampleswere analyzed for δ18O and δ13C by stable isotope-ratiomass spectrometry (SIRMS) and U-Th geochronology bymulti-collector inductively coupled plasma-mass spectrom-etry (MC-ICP-MS) on a parallel leachate/sequential dissolu-tion approach for single and isochron ages (for method seeSupplement). Furthermore, 87Sr / 86Sr isotope signatures foraliquots of the individual U-Th solutions by thermal ioniza-tion mass spectrometry (TIMS, for method details please re-fer to pore-water Sr isotope analyses) were determined. Lipidextracts for biomarker analyses were determined as well.

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5720 S. Geilert et al.: On the formation of hydrothermal vents and cold seeps

From each homogenized carbonate powder sample, analiquot of 10 mg was separated for carbon δ13C and oxy-gen δ18O stable isotope analysis. A fraction of this (approxi-mately 1 mg) was dissolved by water-free phosphoric acid at73 ◦C in a “Carbo-Kiel” (Thermo Fischer Scientific Inc.) on-line carbonate preparation line and measured for carbon andoxygen stable isotope ratios with a MAT 253 mass spectrom-eter (Thermo-Fischer Inc.). The δ13C and δ18O values werecalculated as deviations from the laboratory standard, re-ferred to the PDB scale, and reported in ‰ relative to V-PDB.The external reproducibility was checked by replicate anal-yses of laboratory standards as being better than ±0.04 ‰for δ13C and ±0.1 ‰ for δ18O (1 sd, n= 7) for this sampleset. However, the single measurement uncertainties were sig-nificantly better and the resulting 2 sd (n= 3) for both mainsamples are given in the Supplement Table S5.

Biomarkers were extracted from 4 g of a powdered sampleand were then sequentially extracted with dichloromethane(DCM)/methanol (3/1, v/v), DCM, and n-hexane (ultra-sonication, 20 min). The combined extracts were dried,derivatized using a BSTFA/trimethylchlorosilane mixture(95/5, v/v; 1 h; 40 ◦C), and analyzed by coupled gaschromatography–mass spectrometry (GC-MS; Hinrichs etal., 2000). GC-MS analyses were carried out with a ThermoFisher Trace 1310 GC coupled with a Quantum XLS Ul-tra MS. The instrument was equipped with a PhenomenexZebron ZB 5MS capillary column (30 m, 0.1 µm film thick-ness, inner diameter 0.25 mm). Fractions were injected with-out splits at 270 ◦C. The carrier gas was He (1.5 mL min−1).The GC oven temperature was ramped from 80 ◦C (1 min) to310 ◦C at 5 ◦C min−1 and held for 20 min. Electron ioniza-tion mass spectra were recorded at 70 eV.

3 Results

3.1 Subsurface structure and evidence for sill-relatedfluid mobilization

Seismic profiles showed a wide range of sediment defor-mation (Fig. 2). Seismic amplitude blanking along verticalzones below the seafloor indicated the flow of gaseous porefluids at the North, Central, and Ring seeps (Fig. 2). Un-derneath these locations sediments were deformed, probablydue to sediment mobilization associated with hydrothermalactivity in response to sill intrusion. In contrast the ReferenceSite sediments showed a more or less continuous successionwithout vertical disturbance. At the North Seep, a shallowhigh-amplitude reversed polarity reflector occurred at 50–60 ms. Sill depths were inferred from the seismic profiles at∼ 500 to 600 m below seafloor (m b.s.f.) for the North Seepand with ∼ 350 to 400 m b.s.f. at the other sites, assumingseismic interval velocities of 1600 to 2000 m s−1. Seismicimages suggest that massive disturbance of sediments andvertical pipe structures are related to channeled fluid and/or

Figure 2. Seismic profiles of the North Seep (a), Smoker Site (b)as well as of the Central Seep and Reference Site (c). Seismic sec-tion showing doming above the Central Seep. There are differentphases of onlap starting about 60 ms (maximum deposition) untilabout 15 ms (minimum deposition) or 48 and 12 m b.s.f., respec-tively, assuming a sediment interval velocity of 1600 m s−1.

gas advection caused by sill intrusions (Fig. 2). Faults are in-dicated that may serve as fluid pathways above potential sillintrusions. A closer inspection of the seismic reflectors at theCentral Seep (Fig. 2c) shows onlap onto a doming structure.On the NW flank of the dome, the deepest onlap occurs at60 ms or 48 m b.s.f. (assuming 1600 m s−1 sediment intervalvelocity), whereas on the SE flank, the shallowest onlap oc-curs at 15 ms or 12 m b.s.f.

3.2 Temperature measurements

Heat flow and temperature gradients were measured at theNorth and Central seeps, Reference Site, and Smoker Site(attached to GCs) as well as in transects along the hydrother-mal ridge and rift axis (attached to a temperature lance;Figs. 3 and S2, Table 1). Temperature gradients are shown inFig. S2. The highest heat flows occurred close to the SmokerSite and ranged from 599 to 10 835 mW m−2. Temperaturegradients were also highest at the Smoker Site (∼ 15 K m−1).In contrast, heat flows and temperature gradients in the riftvalley close to the rift axis ranged from 262 to 338 mW m−2

and 0.4 to 0.5 K m−1, respectively. Generally heat flow val-ues decreased with increasing distance to the rift axis with140 mW m−2 at the Reference Site, 113 mW m−2 at the Cen-

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27.5° N

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View

(a)

(b)

North Seep

Central Seep

Reference Site Graben Site

Smoker Site

Figure 3. (a) Heat flow in the vicinity of the northern trough. Notethe different heat flow scale in the enlarged area of the SmokerSite (b).

tral Seep, and 28 mW m−2 at the North Seep. Temperaturegradients were 0.22 K m−1 at the Reference Site, 0.16 K m−1

at the Central Site and 0.14 K m−1 at the North Site.

3.3 Sediment characteristics and sedimentation rates

The sediments were mainly composed of organic-rich di-atomaceous clay, consistent with earlier analyses (e.g., Kast-ner, 1982). At the North Seep, the sediments were composedof homogeneous diatomaceous clay containing rare shellfragments and carbonate concretions. Gas hydrates were dis-covered at 2.5 m b.s.f. Authigenic carbonates were exposedat the seafloor. At the Ring Seep, SW of the North Seep,sediments were predominantly composed of diatomaceousclay. At the Central Seep, located between the North Seepand Smoker Site, sediments were composed of homoge-neous diatomaceous clay intercalated with whitish layers andshell fragments occurring in the shallow sediment (≤ 70 cm).Again, authigenic carbonates were observed on the seafloor.At the Smoker Site, ca. 500 m SE of the hydrothermal ventfield, surface sediments were likewise composed of diatoma-ceous clay with light and dark greyish banding. Traces of bio-turbation were visible in the upper 4 m. At this depth, a sharpcontact defined the transition to the underlying hydrothermaldeposits, which were composed of millimeter-to-centimeter-sized black to grey Fe-rich sulfides (for a detailed descriptionsee Berndt et al., 2016). Within the hydrothermal deposits,brownish to grey clay lenses appeared. At the Slope Site, sed-iments were laminated in the millimeter to centimeter range.The sediment was dominated by diatomaceous clay that con-tained a few ash lenses.

The sedimentation rates ranged from 0.4 m kyr−1 at theSmoker Site to 3.5 m kyr−1 at the North Seep, based on ra-dionuclide measurements (Table 1). Sedimentation rates atall other sites were about 2 m kyr−1.

3.4 Pore water geochemistry

All pore-water data and isotope measurements of 87Sr / 86Srare listed in Tables S1 and S2. Pore water profiles of TA,TH2S, SO4, CH4, NH4, Cl, Mg, and Li are shown in Fig. 4a(GCs) and b (MUCs).

Pore water constituents plotted in Fig. 4 were selectedto characterize variations in organic matter diagenesis, theanaerobic oxidation of methane (AOM), and potential water–rock interactions related to subsurface hydrothermal activ-ity. In general, methane concentrations were elevated at theseep locations and at the slope, thus enhancing AOM. TAand TH2S increased with depth for the North Seep, CentralSeep (only MUC04), and Slope Site, while SO4 was decreas-ing. AOM depths could only be inferred for the North Seepwith ∼ 160 cm and the Slope Site with ∼ 300 cm. NH4 wasonly slightly increasing with depth; higher NH4-levels wereonly found at the Slope Site (Fig. 4). Concentrations of Cl,Mg, and Li did not show significant variations from seawa-ter in shallow sediment depths (MUCs). At greater depths(GCs) some deviations from seawater concentration occurredat the North Seep, Smoker Site, and Slope Site. At the NorthSeep, Mg showed a minor offset at ∼ 150 cm depth, whileat the Smoker Site Mg concentrations increased continu-ously. In GC09 at the Smoker Site, Li concentrations in-creased and Mg concentrations decreased abruptly at a depthof ∼ 400 cm. At the Slope Site, Mg increased slightly be-low 400 cm sediment depth while Li showed a small decreaseabove 400 cm.

Sr concentrations and isotopes are plotted in Fig. 5. Sr con-centrations showed predominantly modern seawater values,except at the North Seep where they strongly decreased. The87Sr / 86Sr isotope ratios also showed predominantly sea-water values (0.709176; Howarth and McArthur, 2004), ex-cept for the Smoker Site, where the isotope ratios decreasedstrongly below the transition between hemipelagic sedimentsand hydrothermal deposits (Fig. 5). The North and Ringseeps as well as the Smoker Site (GC10) showed slight de-creases in 87Sr / 86Sr. The ratios showed a similar depletionto those from the hydrothermal plume (Berndt et al., 2016).

3.5 Hydrocarbon gases, carbon, and hydrogen isotopedata

Concentrations of dissolved hydrocarbons and δ13CCH4 ,δ13CC2H6 , and δDCH4 data are reported in Table S3. Over-all, our pore fluid data showed a large variability inCH4 / (C2H6+C3H8), with ratios between 100 and 10 000and δ13CCH4 between −26.5 ‰ and −88.2 ‰. Gas hydrateδ13CCH4 ranged from −57.9 ‰ to −58.9 ‰. The δ13CC2H6

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5722 S. Geilert et al.: On the formation of hydrothermal vents and cold seeps

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(b)

10-3 10-1 101 103

NH4 (µM)

Figure 4. Pore water profiles of GCs (a) and MUCs (b). For the Central Seep, GC13 is shown here as an example, and geochemical data ofthe remaining cores (GC03, 15) can be found in Table S1. Endmember composition of hydrothermal solutions from Von Damm et al. (1985)and hydrothermal plume geochemical composition from Berndt et al. (2016) are shown in (a) for comparison.

values ranged between −26.1 ‰ and −38.3 ‰ for the NorthSeep and −29.6 ‰ and −37.7 ‰ for the Central Seep. TheδDCH4 values at both seeps ranged between −97 ‰ and−196 ‰, between −196 ‰ and −198 ‰ for the gas hy-drates, between−192 ‰ and−196 ‰ for the Slope Site, andbetween −98 ‰ and −113 ‰ for the hydrothermal plume(VCTD09).

3.6 Water column data

Water column characteristics like temperature, salinity, tur-bidity, and methane concentrations are shown in Fig. 6 andTable S4. Surface waters in the Guaymas Basin showedwarm temperatures of up to 29.5 ◦C (salinity: 34.5 ‰) closeto the Mexican mainland (Slope Site, VCTD07) and up to24.6 ◦C (salinity: 34.6 ‰) in the central basin (Central Seep,VCTD02). With depth, temperatures decreased continuouslyand ranged from 2.8 to 3.0 ◦C (salinity: 34.6 ‰) close to the

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S. Geilert et al.: On the formation of hydrothermal vents and cold seeps 5723

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North Seep (MUC11)Central Seep (MUC04)Ring Seep (MUC05)Reference Site (MUC02)Modern seawater(Howarth & McArthur, 2004)

(a)

(b)

Figure 5. Sr concentrations and 87Sr / 86Sr ratios for GCs (a) andMUCs (b). For comparison, data from the hydrothermal plume(Berndt et al., 2016), the hydrothermal endmember (Von Damm etal., 1985), and modern seawater (Howarth and McArthur, 2004) areshown. Note the different x-axis scales for MUC Sr concentrationand 87Sr / 86Sr ratios.

seafloor (1600–1800 m). Turbidity values were high in thedeep water layer (∼ 1400–1800 m) and indicate a well-mixeddeep basin, also shown by relatively homogeneous tempera-ture and salinity data. Only the water column directly abovethe hydrothermal vent field showed a strongly elevated tem-perature (28.4 ◦C) and salinity (35.1 ‰) (Berndt et al., 2016).Methane concentrations were highest close to the hydrother-mal vent field (up to 400 µM; VCTD09 from Berndt et al.,2016) but still varied in the deep water column of the basinbetween 2 and 28.1 nM (Central Seep, VCTD02 and RingSeep, VCTD01, respectively).

3.7 Authigenic carbonate data

The authigenic carbonate sample (Fig. S1) consisted of 88to 90 % aragonite and 6 to 12 % calcite (Table S5). By theuncertainty related maximum deviation of 1d104 (< 0.01),the XRD spectrum identified calcite with an Mg fractionbelow 3 %, according to Goldsmith et al. (1961). The bulkouter-rim carbonate had an average carbon isotope signa-ture (δ13CV-PDB) of −46.6± 0.2 ‰ and an oxygen isotopesignature (δ18OV-PDB) of 3.7± 0.3 ‰. Inner- core carbonateisotope signatures yielded similar values, with δ13CV-PDB of−44.7±0.4 ‰ and δ18OV-PDB of 3.6±0.1 ‰ (Table S5). Theaverage outer rim 87Sr / 86Sr ratio was 0.709184±0.000027and the inner-core ratio was 0.709176± 0.000003. The U-

Th carbonate dating approach on these authigenic carbonatesimplied formation ages younger than 240 yrs BP.

Lipid extracts obtained from the seep carbonate 56-VgHG-4 (Central Seep) revealed a strong signal of spe-cific prokaryote-derived biomarkers (Fig. S1). These com-pounds encompassed isoprenoid lipids derived from archaea,particularly crocetane, 2,6,10,15,19-pentamethylicosane(-icosenes; PMI, PMI1), archaeol, and sn2-hydroxyarchaeol(see Fig. S1 for structures). In addition, the sample containeda suite of non-isoprenoid 1,2-dialkylglycerolethers (DAGE)of bacterial origin. Typical compounds of planktonic origin,such as sterols, were also present but were low in abundance.

4 Discussion

4.1 Origin of seeping fluids

4.1.1 Smoker Site

The water column above the newly discovered hydrother-mal vent field exhibits elevated CH4 concentrations (upto 400 µM) and pCO2 data (> 6000 µatm) (Berndt et al.,2016). The range of the measured stable isotope signatureof methane (δ13CCH4 between −39 ‰ and −14.9 ‰) andthe helium isotope anomaly (3He / 4He ratio of 10.8×10−6)

clearly indicate the existence of gas exhalations from ther-mogenic organic matter degradation with contributions froma mantle source (see Berndt et al., 2016). These northerntrough hydrothermal fluids are comparable in their gas geo-chemistry to the southern trough (Lupton, 1979; Von Dammet al., 1985; Berndt et al., 2016). However, the highest heatflow values of up to 10 835 mW m−2 measured in this studyare found close to the Smoker Site and are much higher thanthose observed in earlier studies (maximal 2000 mW m−2,Fisher and Becker, 1991). The high heat flow at the SmokerSite even exceeds the more hydrothermally active southerntrough, where heat flow values of 2000 to 9000 mW m−2

were measured (Lonsdale and Becker, 1985; Fisher andBecker, 1991). This might indicate that hydrothermal activityat the northern trough is younger and possibly a more recentprocess compared to the southern trough.

Hydrothermal fluids are typically depleted in Mg and arehighly enriched in fluid-mobile elements like Li caused byhigh-temperature reactions with mafic rocks (here sills) and/or sediments through which they percolate (e.g., Einsele etal., 1980; Gieskes et al., 1982; Kastner, 1982; Von Damm etal., 1985; Lizarralde et al., 2010; Teske et al., 2016). Suchcompositions were reported from DSDP site 477 (Gieskeset al., 1982) and fluids obtained by Alvin dives (Von Dammet al., 1985) (see Fig. 1 for location of Site DSDP 477). Al-though strongly diluted, CTD samples from the hydrothermalplume in the northern trough show this trend (Berndt et al.,2016).

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5724 S. Geilert et al.: On the formation of hydrothermal vents and cold seeps

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North Seep (VCTD03)Central Seep (VCTD2)Ring Seep (VCTD01)Graben Site (CTD01)Smoker Site (VCTD06)Smoker Site (VCTD10)Slope Site (VCTD07)Hydrothermal plume (VCTD09;Berndt et al., 2016)

Figure 6. Water column temperature, salinity, turbidity, and methane concentrations. Note that the upper ∼ 300 m below sea level (b.s.l.)in the turbidity data are not shown for scale matters. VCDT09 and temperature data from VCDT10 are from Berndt et al. (2016), all otherparameters were acquired in this study.

An indication for the presence of hydrothermal fluids inpore waters in the vicinity of the hydrothermal vent field isfound at about 4 m depth in core GC09. Here, positive Li andnegative Mg concentrations (Fig. 4a) are probably causedby the weak admixing of hydrothermal solutions (Gieskeset al., 1982; Hensen et al., 2007). Likewise, 87Sr / 86Sr iso-tope ratios decrease to a value of 0.708949 (Fig. 5) andthus tend towards the 87Sr / 86Sr ratio of the local hydrother-mal endmember (87Sr / 86Sr= 0.7052; Von Damm, 1990).Hydrothermal endmember Li concentrations in the Guay-mas Basin have been reported in a range between 630 and1076 µM (Von Damm et al., 1985) and are 20 to 30× higherthan those measured at the Smoker Site (∼ 34 µM; Fig. 4a,Table S1). Here, hydrothermal fluids account for about 3 %of the mix with seawater (Fig. 7). The sediments in thiscore section also change from diatomaceous clay to uncon-solidated, coarse-grained hydrothermal deposits (Fe-rich sul-fides; see also Sect. 3.3), which may facilitate the circulationof hydrothermal fluids.

Despite the proximity of the remaining GCs and MUCsto the hydrothermal vent field (∼ 500 m distance; temper-atures immediately after retrieval are up to 60 ◦C) typicalpore fluid indicators such as Mg, Li, and 87Sr / 86Sr do notshow major excursions from seawater values (Fig. 4). Simi-larly, NH4, an indicator for a diagenetic or catagenetic break-down of organic matter, is only poorly enriched in sedimentssurrounding the hydrothermal vents (NH4 ≤ 0.3 mM). NH4remains well below the value reported from the southerntrough (20 mM; Von Damm et al., 1985) and the Slope Site(GC07) where 10 mM were already reached at subsurfacedepths of only a few meters (Fig. 4). The pore fluid geo-chemistry around the hydrothermal vent field therefore con-firms that early diagenetic processes are not intense (Fig. 7)and that the shallow sediments are not significantly affectedby hydrothermal fluids. We hypothesize that hydrothermalventing causes a shallow convection cell drawing seawaterthrough the sediments towards the hydrothermal vent field,while the sediments become heated by lateral heat conduc-

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North Seep (GC01)Central Seep (GC03)Smoker Site (GC09)Slope Site (GC07)Guaymas Vent South (Von Damm et al., 1985, 1990)Hydrothermal plume(Berndt et al., 2016)Deep-sourced cold seeps(Aloisi et al., 2004;Hensen et al., 2007)

Intenseorganic matterdegradation

Deep thermogenic/diageneticsignal

Figure 7. NH4 (µM) versus Li concentrations (µM) of GuaymasBasin cold seeps (North, Central) and the Smoker Site. Deep fluidsfrom the Smoker Site (GC09) mix with hydrothermal fluids with ashare of ∼ 3 %. The mixing line has been calculated as follows;xmix = xphase1× f1+ xphase2× f2, (R1)with f1+ f2 = 1. Endmember 1 is the Guaymas Vent South (VonDamm, 1990; Von Damm et al., 1985) and endmember 2 is Guay-mas North Seep. For comparison, Guaymas hydrothermal endmem-ber fluid composition (Von Damm, 1990; Von Damm et al., 1985),hydrothermal plume fluid composition (Berndt et al., 2016), Guay-mas slope sediments (GC07), and deep-sourced cold seeps (Aloisiet al., 2004; Hensen et al., 2007) are shown.

tion (cf. Gamo et al., 1991; Henry et al., 1996; Kinoshita andYamano, 1997).

The diatomaceous clay might act as a seal to fluids mi-grating upwards, which are channeled to the catchment areaof the rising hydrothermal fluids of the hydrothermal vent

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S. Geilert et al.: On the formation of hydrothermal vents and cold seeps 5725

field (see also Fig. 4 in Berndt et al., 2016). The geochemi-cal composition of these fluids is likely influenced by a hightemperature-chemical alteration of the sediment caused bythe intruded sills (Fig. 2b). However, shallower pore fluidsof surface sediments at the Smoker Site (i.e., 0–4 m) are notaffected much by contributions from these fluids and showpredominantly ambient diagenetic fluid signatures.

4.1.2 Cold seeps

The selection of sampling sites at presumed seep locationswas based on existing published data (Lizarralde et al., 2010)and information from seismic records (Fig. 2). Seismic am-plitude blanking along vertical zones below the seafloor in-dicates active fluid and/ or gas conduits at the North andCentral seeps. Given that sill intrusions and related high-temperature alteration of sediments are driving the seepage,the expectation was to find deeply sourced (average sill depth∼ 400 m) fluids with a typical geochemical signature analo-gous to findings at hydrothermal vents in the Guaymas Basin(Von Damm et al., 1985; Von Damm, 1990; Berndt et al.,2016). Such characteristics are, e.g., a high concentrationof thermogenic hydrocarbon gases formed by organic-matterdegradation, enrichments in NH4, a depletion in Mg, and astrong enrichment in fluid-mobile tracers like Li and B (e.g.,Aloisi et al., 2004; Scholz et al., 2009). The hydrocarbonformation caused by abiogenic processes plays only a minorrole in the hydrothermal vent field (McDermott et al., 2015;discussion in Berndt et al., 2016).

Samples obtained using a video-guided MUC revealed thehighest methane concentrations at the North, Central, andRing seeps (Fig. 4b). In conjunction with visual evidence(abundant chemosynthetic biological communities), this con-firms that we have hit active seepage areas during our sam-pling campaign. At the two most active sites, North and Cen-tral, high methane levels are accompanied by a significantdrop in sulfate and an increase in TA and TH2S, providingevidence for AOM, according to the following net reaction;

CH4+ SO2−4 → HCO−3 + HS−+ H2O (R2)

(e.g., Nauhaus et al., 2005; see Wegener et al., 2016 for arecent update).

These pore-water trends are even more pronounced inGC01 (North) where the AOM zone was completely pene-trated and gas hydrate was found at about 2.5 m b.s.f. Un-fortunately, GCs from similarly active sites could not be ob-tained from the Central and Ring seeps, mainly because ofpatchiness of seepage spots and the widespread authigenicmineralizations at the seafloor preventing sufficient penetra-tion. Nevertheless, active methane seepage at all three in-vestigated sites is evident. The methane flux is, however,not accompanied by any significant excursion of pore-waterconstituents typical for deeply sourced, high-temperaturesediment–water interactions (e.g., Mg, Cl, Li). Also, Sr con-centrations show seawater values at all seep sites (Fig. 5),

except for the North Seep, where values drop together withCa due to co-precipitation during carbonate formation. The87Sr / 86Sr ratios show predominantly seawater signatures aswell (Fig. 5, Table S2). Similarly, low NH4 concentrations of< 1 mM indicate a low intensity of organic matter decompo-sition (as discussed in Sect. 4.1.1). Taken together, our datashow that, with the exception of methane and sulfate, thepore water corresponds to ambient diagenetic conditions thatare typically met in this shallow subsurface depth. An expla-nation for the decoupling of methane levels and pore-watercomposition is that only methane is rising to the seafloor as afree gas. This assumption requires a closer look at the com-position of dissolved hydrocarbons in general, which is givenbelow.

4.2 Origin of hydrocarbon gases

4.2.1 Alteration effects

The origin of hydrocarbon gases can be deciphered by plot-ting CH4 / (C2H6+C3H8) ratios versus δ13CCH4 data in amodified Bernard diagram (Schmidt et al., 2005 and lit-erature therein) (Fig. 8a) and δ13CCH4 versus δDCH4 afterWhiticar (1999) and Welhan (1988) (Fig. 8b). Most of themeasured stable isotope data of pore-water methane indi-cate a microbial origin or a mixed microbial and thermo-genic origin (Fig. 8). By contrast, hydrocarbons venting atthe hydrothermal vent field reflect a mixture of thermogenicmethane and abiogenic methane derived from water–rock in-teractions (Berndt et al., 2016).

It has to be considered though that, except of three samplesfrom the North Seep, all δ13CCH4 measurements were per-formed on samples located above the AOM zone (see Fig. 4).This implies that the upward-rising methane has likely under-gone fractionation due to methane oxidation by sulfate in theAOM zone underneath. AOM enriches DIC in 12C and re-sults in progressively increasing δ13CCH4 values in the resid-ual methane (Whiticar, 1999). Considering the δ13CCH4 atthe Slope Site as a microbial endmember composition forthe Guaymas Basin (Fig. 8a), most of the data fall on calcu-lated fractionation lines for AOM, following a Rayleigh trend(Whiticar, 1999). Methane sampled close to the Smoker Site(MUC15) is obviously also affected by AOM (Fig. 8a). Thisis in line with recent studies on hydrothermal sediments ofthe southern trough of the Guaymas Basin, where bacterialand archaeal communities catalyze the oxidation of methaneand higher hydrocarbons, shifting δ13CCH4 values to heaviersignatures (Dowell et al., 2016).

The origin of methane and oxidation effects can fur-ther be identified in the δ13CCH4 versus δDCH4 plot afterWhiticar (1999) and Welhan (1988) (Fig. 8b). Slope Sitesamples plot in the field of microbial CO2 reduction whilehydrothermal plume samples plot in the thermogenic field.One sample even points to a mantle signature and thus showspotential endmember isotope signatures (Berndt et al., 2016).

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5726 S. Geilert et al.: On the formation of hydrothermal vents and cold seeps

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Figure 8. Hydrocarbon, δ13CCH4 , and δD isotope data for Guaymas Basin seep sites and Smoker and Slope sites. Hydrothermal plume dataare shown for comparison. Note that hydrocarbon and δ13CCH4 data are from Berndt et al. (2016). (a) CH4 / (C2H6+C3H8) ratios versusδ13CCH4 data are shown after a modified Bernard diagram (Schmidt et al., 2005). Pale symbols indicate samples above the AOM zone.Rayleigh fractionation lines show the effect of (microbial) methane oxidation, and labels indicate the residual methane in %. (b) Carbon(δ13CCH4) and hydrogen (δDCH4) isotope data after Whiticar (1999) and (Welhan, 1988). Pale symbols (Central Seep – MUC04) indicatesamples above AOM zone.

North Seep samples (pore fluids and gas hydrates) plot in themixing region while samples from the Central Seep clearlyshift away from the microbial field and are considered to beaffected by bacterial oxidation (Whiticar, 1999).

Considering the methane below the AOM as being un-altered, three North Seep samples and the majority of theSlope Site samples show a clear microbial source of methane(Fig. 8a). All other samples appear to be affected by ma-jor oxidation following a Rayleigh fractionation process andshow that only a fraction between 2 % (MUC04, CentralSeep) and 0.05 % (GC15, Central Seep) remains as unoxi-dized methane (Fig. 8a).

4.2.2 Origin of unaltered samples

The δ13CCH4 versus δDCH4 plot of the unaltered North Seepsamples suggests a mixing of microbial and thermogenicmethane (Fig. 8b). Similar signals have also been observedat Hydrate Ridge (Milkov et al., 2005) and seem to be a com-mon phenomenon in hydrothermal and cold-seep-affectedsediments. In a few samples from the North and Central seepsethane concentrations have been high enough to measure sta-ble carbon isotopes, and the δ13CC2H6 values point to a ther-mogenic origin (Table S3).

4.3 Timing of active (thermogenic) methane release

4.3.1 Seep site geochemistry

Based on our data set, no deep-sourced fluid is currentlymigrating upwards at the cold seeps investigated (comparedeep-sourced seepage sites from the Gulf of Cadiz in Fig. 7).Hence, in terms of the original hypothesis that fluid emana-tion is directly linked to recent sill intrusions, these cold seepsites cannot be considered active as claimed by Lizarraldeet al. (2010). These authors argued that thermogenic car-bon is currently released up to 50 km away from the riftaxis, causing a maximum carbon flux of 240 kt C yr−1. Fur-ther, Lizarralde et al. (2010) showed temperature anomalies,high methane concentrations, and helium isotopic anoma-lies in the water column potentially indicative of a mag-matic source. These anomalies were detected in close vicin-ity to bacterial mats, tubeworms, and authigenic carbonates,situated above areas of sill intrusions. Comparable struc-tures have been identified in this study by video-guidedMUCs and seismic data (Fig. 2). Our detailed results onpore fluid, water column, and gas geochemistry now showthat most methane was of microbial origin (Fig. 8) and onlytraces of thermogenic methane were found up to ∼ 20 kmoff axis (North Seep). Even pore fluids taken close to thehydrothermal vent field are dominated by shallow microbialdegradation processes, indicating that the hydrothermal fluid

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S. Geilert et al.: On the formation of hydrothermal vents and cold seeps 5727

flow in the Guaymas Basin is rather localized and boundto focused fluid pathways. The temperature and chemicalanomalies detected by Lizarralde et al. (2010) might alsoarise from the deep water layer in the Guaymas Basin it-self, which is influenced by hydrothermal fluids (Campbelland Gieskes, 1984). Hydrothermal activity in the GuaymasBasin produces hydrothermal plumes, rise 100–300 m aboveseafloor and spread out along density gradients throughoutthe basin (Campbell and Gieskes, 1984). Our results never-theless show that the Guaymas Basin has a well-mixed bot-tom seawater layer, with temperatures ranging between 2.8and 3.9 ◦C in > 1000 m depth and off-axis methane concen-trations that vary quite considerably (e.g., 6 to 28 nM at RingSeep, Fig. 9). These bottom seawater variabilities are biggerthan the reported anomalies by Lizarralde et al. (2010) andmight indicate that thermogenic methane release might notbe as widespread as previously suspected.

Pore fluids taken in a transect up to ∼ 30 km away fromthe rift axis show no evidence for high-T reactions (Figs. 4,7). We can still not exclude the possibility that thermogenicmethane is released in other areas of the basin, but the lackof evidence for high temperature geochemical processes atour sites is evident and clearly contradicts the conclusionsdrawn by Lizarralde et al. (2010). Our findings suggest that aprojection of the thermogenic methane release based on thenumber of detected sills (Lizarralde et al., 2010) representsa maximum estimate, as it neither considers the time of theemplacement of a sill nor the lifetime of such magmatic sys-tems. Today, shallow microbial degradation processes deter-mine pore fluid signatures (Figs. 4, 8). Whereas high temper-ature thermogenic reactions have certainly been active duringsill emplacement and once released large amounts of carbon,these processes have apparently ceased. However, pipe struc-tures may still act as high-permeability pathways and facili-tate the advection of gas. As a result, small amounts of ther-mogenic carbon might be released as reflected by the signa-tures of δ13CCH4 and the thermogenic δ13CC2H6 isotope dataat the North and Central seeps. However, present methaneadvection rates are slow (probably < 1 cm yr−1), as observedby low methane gradients in the pore fluid profiles (Fig. 4).These conditions favor an effective turnover of CH4 to bicar-bonate and authigenic carbonates by AOM (Wallmann et al.,2006; Karaca et al., 2010).

4.3.2 Origin of the authigenic carbonate

The porous authigenic carbonate block recovered from theseafloor at the Central Seep can preserve long-term informa-tion about seepage in this area. The predominant biomark-ers found in the seep carbonate from the Central Site (56-VgHG-4) are consistent with microbial consortia perform-ing AOM. In particular, high abundances of crocetane andsn2-hydroxyarchaeol indicate major contributions from themethanotrophic archaea of the ANME-2 cluster, whereasDAGE originate from syntrophic sulfate-reducing bacteria,

0 10 20 30Distance to rift axis (km)

100

102

104

CH

4 (nM

)

2.8

3.6

4.4

Tem

pera

ture

(°C

)

NorthSiteGraben

Site

SmokerSite

CentralSite

RingSite

(a)

(b)

Figure 9. Water column CH4 (a) and temperature (b) at cold seepsas well as the Smoker and Graben sites relative to the rift axis.

probably of the Desulfosarcina–Desulfococcus group (Blu-menberg et al., 2004; Niemann and Elvert, 2008). These con-sortia gain energy from AOM, with sulfate as the final elec-tron acceptor (see Eq. 2).

At the Central Seep, the increase in TA due to theAOM reaction plausibly explains the precipitation of isotopi-cally depleted authigenic carbonates. Particularly, ANME-2biomarkers have been reported in association with abundantfibrous, often botryoidal aragonite cements (Leefmann et al.,2008), which is in line with the observations made at theCentral Seep (see Sect. 3.3). Moreover, the high abundanceof ANME-2 indicates that seep carbonate formation tookplace under high sulfate concentrations and strong advectivemethane flow, but there were no elevated water temperatures(c.f. Nauhaus et al., 2005; Peckmann et al., 2009; Timmers etal., 2015). Minor amounts of typical marine sterols also showthat the seep carbonates also captured detritus from the sur-rounding sediment and water column during their ongoingcementation.

The bulk carbonate carbon isotope signature (δ13CV-PDB =

−46.6 ‰) overlaps with the shallow, heavy δ13CCH4 values(−27.5 ‰ and −48.6 ‰) in the pore fluids at the CentralSeep and confirms a dominant AOM signature with a minorplanktonic and potentially δ13C-diluting background signal.The oxygen isotopes point to a low formation temperature ofabout 3 ◦C, consistent with a precipitation in bottom waters(2.8 to 3.0 ◦C (Figs. 6, 9; Table S4). The 87Sr / 86Sr analysessupport this assumption by values within uncertainty identi-cal to modern seawater. U-Th carbonate dating provide agesyounger than 240 yrs BP. In summary, authigenic carbon-

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5728 S. Geilert et al.: On the formation of hydrothermal vents and cold seeps

ates originate from shallow methane and were subrecentlyformed in ambient seawater.

4.3.3 Timing of off-axis hydrothermal activity

The seismic data taken across the seep locations indicate thatthe disrupted sediment layers do not reach the sediment sur-face (Fig. 2a, c). This implies that fluid mobilization ceasedat some time before the uppermost sediment layers were de-posited. The doming above the Central Seep provides someclues on the timing of fluid migration (Fig. 2c). Assumingthat the doming is the result of buoyancy-related uplift (Kochet al., 2015), it represents the time when intrusion-related gasreached the seafloor. Assuming further a sedimentation rateof 1.7 m per 1000 years (Central Seep; Table 1) and maximaand minima deposition depths of 48 and 12 m below seafloor,respectively (see Fig. 2c), this would imply that most of thegas reached the seafloor between 28 and 7 kyr ago. Evenat maxima and minima sedimentation rates of 3.5 m (NorthSeep) and 0.5 m (Ring Seep) per 1000 years, the gas flowwould have ceased between 14 and 3 kyr ago at the earliestand between 96 and 24 kyr ago at the latest. Accordingly, thisfinding further supports the results of the pore fluid and gasgeochemistry, which show no sign of active fluid flow froma depth at the cold seep sites in the northern Guaymas Basin.

We agree with Lizarralde et al. (2010) that hydrothermalactivity in the Guaymas Basin is an important driver for CH4(and CO2) emissions into bottom waters. However, our dataset shows that there is no deep fluid advection at the inves-tigated sites. Our interpretation is that hydrothermal activityat these off-axis locations has ceased and previously formedpathways seem to mediate the advection of biogenic gas atpresent. It is not unlikely that seep-induced, hydrothermal ac-tivity is still ongoing in other places than those investigated inthis study, but in order to provide more accurate predictionsfor (thermogenic) carbon fluxes and the potential impact onthe climate, sill emplacement mechanisms need to be betterconstrained. Apart from their spatial distribution, the mostimportant and currently unknown factors are the determina-tion of the time of their emplacement and the longevity of thesill-systems that require further investigation.

5 Conclusions

Magmatic sill intrusions into organic-rich sediments can po-tentially release large amounts of carbon into the water col-umn and atmosphere and are therefore considered potentialtrigger mechanisms for rapid climate change, e.g., duringthe PETM. Sill-induced hydrothermalism has been reportedalong the ridge axis in the Guaymas Basin (von Damm etal., 1985; Berndt et al., 2016) and the widespread occur-rence of sills and fluid escape features within the basin hasbeen used to estimate the related carbon release (Lizarraldeet al., 2010). Our investigations of off-axis methane seeps

in the Guaymas Basin demonstrate that there are no indi-cations for hydrothermal activity away from ridge axis atpresent. These conclusions are mainly based on the lack ofgeochemical signals from high temperature alteration pro-cesses and the CH4 predominantly originating from micro-bial degradation. We suggest that hydrothermal circulationhas, based on seismic records and dating of authigenic car-bonates, largely ceased at the investigated sites several thou-sands of years ago. This finding underlines that the vigorousventing, as presently observed at the ridge axis, is a very ef-fective way to release sedimentary carbon into the water col-umn but must be considered a very short-lived process in ageological sense. Hence, a more comprehensive understand-ing of these hydrothermal systems with respect to their cli-matic relevance requires a better knowledge of the controlmechanisms and their longevity.

Data availability. All research data is accessible in the supplementof this manuscript.

The Supplement related to this article is available onlineat https://doi.org/10.5194/bg-15-5715-2018-supplement.

Author contributions. SG, CH, MS, VL, FS, and AF were involvedin research cruise SO241 and carried out sampling, sample prepara-tion, and measurements. MD conducted temperature measurements.ML took samples for Pb measurements, LD processed the samples,and CCS performed the analyses. SS carried out the stable hydro-gen isotope measurements. VT performed the biomarker analyses.SS and CB were responsible for the seismic data recording. All au-thors discussed the results and commented on the manuscript. CHand CB planned the study and were responsible for the planning ofthe research cruise.

Competing interests. The authors declare that they have no conflictof interest.

Acknowledgements. This work was undertaken within the MAKSproject funded by the German Ministry of Science and Education(BMBF). We thank the master and crew of the R/V Sonne fortheir support during the SO241 cruise. Further thanks goes toRegina Surberg, Bettina Domeyer, and Anke Bleyer for analyticalsupport during the cruise and on shore. We greatly appreciate thesupport from Ana Kolevica, Tyler Goepfert, Sebastian Fessler,Andrea Bodenbinder, Yan Shen, and Jutta Heinze for onshoreanalyses. Additional support of this work was provided by EU-COST Action ES1301 “FLOWS” (https://www.flows-cost.eu, lastaccess: 25 September 2018). We would also like to thank the editorHelge Niemann and two anonymous reviewers for their commentsand constructive reviews.

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S. Geilert et al.: On the formation of hydrothermal vents and cold seeps 5729

The article processing charges for this open-accesspublication were covered by a ResearchCentre of the Helmholtz Association.

Edited by: Helge NiemannReviewed by: two anonymous referees

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